The present invention relates generally to gas turbine engines, and, more specifically, to an improved gas turbine engine rotor assembly which incorporates an inertial damper for damping selected vibrations transmitted from one portion of the rotor assembly to another portion thereof.
Gas turbine engines have a rotor assembly mounted for rotation within a stator. The rotor assembly typically has a plurality of axially spaced rows of circumferentially spaced blades mounted on a main rotor shaft. In the compressor stage of such rotor assembly, the blades of each row progressively decrease in size in the direction of flow, that is, the blades of each row are smaller than the blades of the immediately adjacent upstream row.
A bevel gear is typically mounted on the main rotor shaft, and is continuously engaged with a cooperative auxiliary bevel gear located at the distal end of an auxiliary shaft. This auxiliary shaft is commonly known as a power-take-off shaft which often has its longitudinal axis arranged so as be perpendicular, or at least oblique, to the axis of the main shaft. A starter is operatively coupled to the auxiliary shaft. Thus, for example, during engine start-up, the starter causes the auxiliary shaft to rotate up to about 7,000 rpm. Such motion is transmitted through the auxiliary and main shaft bevel gears, to cause the rotor assembly to rotate. After the engine has been started, the powered rotation of the main shaft drives the rotation of the auxiliary shaft. In this mode, the starter is conventionally disconnected and an alternator which is connected to the shaft supplies electrical power to the system of which the engine is a part. Hence, the auxiliary shaft remains mechanically coupled with the main rotor shaft throughout the entire operating speed range of the main shaft which typically extends up to about 46,000 rpm.
In certain engine configurations, it was observed on strain gauges that the fifth-stage compressor blades suffered from excessive vibratory stress levels at or near a resonant frequency during engine start-up. It was noted that the stress due to resonant frequencies of such blades had an apparent relationship to the number of teeth in the rotor assembly bevel gear. More particularly, it was observed that the fifth-stage blades were being excited at "38/rev" (i.e., 38 times the rotor speed), which number coincided with the number of teeth in the rotor assembly bevel gear. From this observation, it was deduced that the vibrational excitation of the fifth-stage blades was attributable to a source of vibrations transmitted from the auxiliary gear to the rotor assembly bevel gear, and through torsional vibration of the rotor shaft to the fifth-stage of the compressor.